1. Field of the Invention
The present invention is directed to high resolution lithography for the fabrication of nano-electronic devices and, more particularly, the manufacture and use of clear phase shifting masks for lithographic processes that enhance the bright-peak of the corresponding aerial image of the mask.
2. Description of the Related Art
Semiconductor computer chips typically include millions of devices fabricated on a single chip. These devices are defined at least in part by linear features or nodes that can be imaged on a wafer of the chip using a photolithographic process, typically comprising a number of steps for depositing multiple layers of materials. For example, trench features are often imaged on the wafer and then filled with metal to provide conductive traces on the chip.
The speed of semiconductor computer chips, such as the Intel Pentium(copyright) chip is dependent on the width of these linear features or nodes imaged on the associated wafer during production. As nodes are made smaller, more nodes can be placed on a chip and the processing speed increases. This technology has developed rapidly. For example, the Pentium(copyright) 3 chip has a nominal node size of approximately 180 nanometers (nm) and the Pentium(copyright) 4 chip has a nominal node size of approximately 120 nm.
A favored method for making these small nodes on chips includes placing a mask above a target (i.e., the chip) which comprises, for example, a silicon or GaAs substrate having a photoresist disposed thereon, and exposing the photoresist to light in a photolithographic technique. Notably, when writing features to a target in this fashion, the lithography process typically generates two types of images, an aerial image (i.e., the radiation intensity of the target surface), and a latent image (i.e., the image recorded in the target photoresist.)
With lithography techniques, the wavelength of the light utilized to form the image on the target photoresist imposes a fundamental limit on the achievable image definition or resolution, i.e., the minimum node width that can be imaged. One limiting factor in making smaller nodes has to do with diffraction, which is an unwanted side effect of the lithography production process. In particular, the image resolution is limited by diffraction of the light at the edges of the features of the masks through which the light is projected. To lessen the negative effects associated with diffraction, and thus allow the creation of smaller nano-electronic structures that are obtainable using visible or UV optical systems, sources that generate short-wave length light rays (e.g., X-rays) are being implemented.
Moreover, as the distance between the mask and the wafer increases, the diffraction of the light waves increases, so the feature produced on the chip is slightly larger than the feature on the mask. Therefore, typically, the smaller separation or gap between the mask and the photoresist, the less diffraction.
Ideally, you would eliminate diffraction by placing the mask directly on the target surface (a technique known as xe2x80x9ccontact printingxe2x80x9d), but for several reasons this is not practical. For instance, the mask is fragile, and the photoresist may be sticky, so removing the mask from the surface after processing becomes difficult. Also, the additional time required in process (due to, for example, using extra care when coupling/separating the mask from the substrate) would decrease yield, as well as increase cost per part. In addition, the target and the mask both may have some amount of curvature, which makes it difficult to couple them and completely eliminate the gap.
Therefore, to further minimize the negative effects associated with diffraction, phase shifting masks (PSMs) may be used. PSMs enhance the imaging resolution of features to be printed on a wafer surface by exploiting a phase difference (e.g., a xcfx80-phase difference) between a phase shifting region and an open region both residing on the surface of a mask membrane. In particular, the phase difference causes interference between phase shifted and non-phase shifted radiation when writing features using the phase shifting mask. This interference occurs on both sides of an edge of the phase shifting regions, and can be observed by analyzing the corresponding aerial image. Typically, the image includes a xe2x80x9cbright peakxe2x80x9d and a xe2x80x9cdark peakxe2x80x9d associated with the edge.
In PSMs, two types of phase shift regions are typically employed, clear phase shift regions, and attenuating phase shift regions. Existing clear PSMs focus on compensating for diffraction by using xe2x80x9cdark peakxe2x80x9d interference waves to image the mask pattern onto the photoresist.
Note that a clear PSM is characterized by comprising phase shifting material that does not substantially attenuate the source radiation (e.g., X-rays) passing therethrough. To the contrary, attenuating PSMs typically comprise an attenuator that absorbs a significant portion of the radiation impinging thereon, thus generating the desired phase shift of that portion of the radiation passing therethrough. A suitable heavy metal absorber, such as Gold (Au) may be made of Tungsten (W) having an appropriate thickness. A drawback associated with attenuating PSMs is that they typically reduce the radiation intensity when imaging mask features. By providing more power to the wafer, clear PSMs require shorter exposure times, and thus facilitate greater wafer throughput.
A typical phase mask and its associated aerial image are shown in FIGS. 1A and 1B, respectively, for the case in which phase X-ray lithography (PXRL) is employed. A mask 10 includes a carrier or membrane 12 having generally planar top and bottom surfaces 14, 16, respectively. Membrane 12 supports a phase shift region 18 defining an edge 20 and is made of a suitable material such as polymethyl-methacrylate (PMMA) having a thickness of about 2.5 to 3 xcexcm such that it provides the desired half wavelength (i.e., xe2x80x9cxcfx80xe2x80x9d) phase shift for a corresponding band of X-ray photons of approximately 1.0 nm in wavelength. Edge 20 defines a boundary line between phase shift region 18 and an open region 21 supported by membrane 12. In this case, membrane 12 is preferably made of silicon nitride (Si3N4) which is sufficiently thin not to absorb greatly the incoming radiation flux (e.g., 22 in FIG. 1A). Further, the gap between the mask and the target resist (not shown) is typically about 15 xcexcm or less.
An aerial image 24 produced by this arrangement is shown in FIG. 1B and is plotted as Normalized Intensity versus Dimension (nm) on the target. Image 24 includes a bright peak 32 and a dark peak 26. In known PXRL systems, dark peak interference is used to compensate for the diffraction at the mask edge, and thus it is the dark peak 26 that is used to image the feature. More particularly, when imaging a node to the target, destructive interference of the spatially coherent phase shifted and non-phase shifted X-rays results in their fields canceling near the boundary line 20 so that the intensity of X-ray energy absorbed by the target photoresist reaches a minimum at a point 26 (i.e., the dark peak) which lies generally on boundary line 20. Preferably, the energy absorption represented by point 26 is below the exposure threshold energy level E1, of the resist; that is, those areas of the resist which receive photon energy below the level of the line E1 illustrated in FIG. 1B will not, for a positive resist, be dissolvable by a developer solvent. The developer will, however, dissolve the regions of the target photoresist to the left of a point 28 at which the threshold E1 intersects the exposure level curve 24 and to the right of a point 30 at which the threshold E1 intersects curve 24. For a more detailed analysis of this technique, see U.S. Pat. No. 5,187,726 issued on Feb. 16, 1993, to Wisconsin Alumni Research Foundation. This technique typically uses the dark peak 26 to image lines in positive-tone resists, or trenches in negative-tone resist.
As mentioned previously, aerial image 24 also includes a bright peak 32. When using a photoresist having a threshold level E2, energy absorption by the resist that is greater than E2 could theoretically be used to image nodes. However, the bright peak 32 is not normally used because its intensity is not sufficiently higher than the dark peak 26, and its contrast is poorer. If, however, these two limitations could be obviated, it would be possible to use the bright peak for imaging as well.
Moreover, typical phase shifting schemes involving a PSM are alternating aperture PSM, attenuated PSM, or clear PSM. As discussed above, X-rays have been employed with the conventional one-to-one masks such as these to circumvent some diffraction effects because of the shorter wavelength of the X-rays. However, all these phase shifting schemes, including the application with X-rays, have been limited to one-dimensional resolution enhancement. For two-dimensional patterns, such as contact holes (or vias), conventional phase shifting alone is not a viable solution.
As a result, the field of semiconductor fabrication using lithography is in need of an apparatus and method that overcomes both the limitations associated with using the dark peak of an aerial image to image nodes in a target, as well as overcome the limitations associated with narrow mask-to-wafer gaps. Further, such a system should be capable of imaging both one-dimensional and two-dimensional patterns without compromising production efficiency and reliability.
The preferred embodiment is directed to a clear phase shifting mask and method of use that specifically enhances the bright peak of the aerial image generated by the mask by appropriately positioning two adjacent phase shifting edges so that they give rise to constructive interference between the bright peaks associated with the individual edges. As a result, the written mask feature can be larger than the printed feature by a factor of 3 to 5. In addition, this relaxes the constraints on writing small features on the mask. Moreover, the constraints on the gap distance between the mask-to-wafer are lessened. In fact, in the preferred embodiment, gaps are preferably larger than those currently used in X-ray lithography utilizing PSMs, thus further facilitating efficient production of reliable chips including nano-electronic devices. Also, the preferred embodiment is adapted to imaging both one-dimensional and two-dimensional patterns using appropriately positioned phase shift regions. Note that the terms xe2x80x9cone-dimensionalxe2x80x9d and xe2x80x9ctwo-dimensionalxe2x80x9d are herein used to refer to the number of critical dimensions (CDs) associated with the nanostructure being produced; for example, trenches are characterized as having one CD, and therefore are referred to as one-dimensional, while a contact hole is a two-dimensional pattern.
According to a first aspect of the preferred embodiment, an X-ray phase mask includes a membrane having generally planar top and bottom surfaces, the membrane being substantially transparent to X-rays. In addition, the mask includes a pair of clear phase shift regions supported on the membrane, each of the regions defining a corresponding edge. During an imaging operation, the mask generates an aerial image defining, for each of the corresponding edges, an edge bright peak and an edge dark peak, the edge bright peaks constructively interfering at a certain distance from the mask to form an enhanced bright peak.
In another aspect of the preferred embodiment, the edges of the clear phase shift regions are separated by a perpendicular distance that is based on the effective wavelength of the X-rays. The perpendicular distance is between about 100 nm and 300 nm when the effective wavelength is about 0.9 nm. Moreover, in the preferred embodiment an image bright peak is used to produce a structure having a critical dimension less than or equal to 50 nm.
According to another aspect of the preferred embodiment, a phase mask includes a membrane having generally planar top and bottom surfaces, and a phase shift material supported by the membrane and defining an open region, wherein the open region defines a feature of the mask. During an imaging operation, radiation propagating through the mask constructively interferes to produce an image bright peak that images the feature in a target photoresist.
According to a still further aspect of the preferred embodiment, a method of producing a nanostructure by lithography includes providing a phase shifting mask defining a feature, and a target positioned parallel to the mask and separated therefrom by a gap. Next, the method includes exposing the phase shifting mask and the target to a beam of radiation such that the corresponding aerial image includes an image bright-peak. The image bright-peak is then used to image the feature on the target.
According to a further aspect of the preferred embodiment, a phase shifting mask for imaging a feature on a target includes a membrane having generally planar top and bottom surfaces. In addition, the mask has first and second phase shift regions supported on the membrane that define corresponding first and second edges. Moreover, the first and second edges are separated by a distance selected so that an aerial image generated by the mask during an imaging operation includes corresponding first and second bright peaks that constructively interfere, where the first and second bright peaks corresponding to the first and second edges.
In yet another aspect of the preferred embodiment, a phase shifting mask that images a two-dimensional feature on a target includes a membrane and a clear phase shifting material supported by the membrane. The phase shifting material defines an aperture that includes two pairs of opposed edges, each pair being separated by an associated perpendicular distance. During an imaging operation, the mask generates an aerial image comprising an edge bright peak associated with each edge Moreover, each associated perpendicular distance is selected to facilitate constructive interference of the edge bright peaks to produce an image bright peak that is used to image the two-dimensional feature.
According to a still further aspect of the preferred embodiment, a method of imaging a two-dimensional mask feature by lithography includes providing a phase shifting mask defining a two-dimensional feature and a target positioned parallel to the mask and separated therefrom by a gap. Next, the method includes exposing the phase shifting mask and the target to a beam of radiation such that the corresponding aerial image includes an image bright-peak that is used to image the two-dimensional feature on the target.
According to another aspect of the preferred embodiment, the aperture is defined by two pairs of opposed clear xcfx80-phase shift regions, each phase shift region defining a corresponding edge, the corresponding edges of each pair being generally parallel to each other and separated by a distance.
These and other objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.